add_key.2: srcfix: FIXMEs

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/add_key.2

@@ -49,6 +49,9 @@ and
 then, if the key type supports it, that key will be updated rather than a new
 key being created; if not, a new key will be created and it will displace the
 link to the extant key from the keyring.
+.\" FIXME In the previous paragraph, what is the significance of the fact
+.\" that in some cases the existing key is updated versus in other cases a
+.\" new key is created? Is it just that the key ID will be different?
 .P
 The destination
 .I keyring
@@ -135,6 +138,7 @@ For this key type,
 the contents can be saved in a tmpfs filesystem and
 thus swapped out to disk when memory pressure is high.
 This key type is useful for tasks such as holding Kerberos ticket caches.
+.\" FIXME Are there any other key tpes available for user-space use?
 .SH RETURN VALUE
 On success,
 .BR add_key ()
@@ -142,6 +146,9 @@ returns the serial number of the key it created or updated.
 On error, \-1 is returned and
 .I errno
 is set to indicate the cause of the error.
+.\" FIXME Is there any difference between the terms "key serial number"
+.\" and "key ID"? What about "special key IDs"? (Can one equally talk
+.\" of a "special key serial number"? 
 .SH ERRORS
 .TP
 .B EACCES